For example, multistage centrifugal compressors are known as a type of centrifugal rotating machine, and an example of the multistage centrifugal compressor is disclosed in Patent Literature 1. Patent Literature 1 discloses a compressor that includes a U-shaped cross-section portion from which a working gas compressed at a first stage impeller and a second stage impeller is discharged, a return flow channel portion in which the working gas after passing through the U-shaped cross-section portion joins with an intermediate stage injection flow suctioned from an intermediate stage injection nozzle and flows radially inward, and a third stage impeller to which the working gas (working gas joined with the intermediate stage injection flow) of the flow directed into an axial direction from a radially inward direction is supplied.
Suction of the intermediate stage injection flow is applied to a compressor used in a refrigeration cycle or the like and is intended to adjust the flow rate required for the cycle.